[1998]. They designed a compartmental liquid sampler, which was inserted into a horizontal borehole, to collect infiltrating solution at multiple locations. They estimated the amount of water flowing through the fractures and imbibing into the matrix across the fracture walls and determined that under variable moisture content conditions the fracture aperture, roughness, and flow channels varied with time. They also noted that under field conditions the effective fracture aperture was more than 1 order of magnitude smaller than that expected based on laboratory conditions. Or and Ghezzehei [2000] studied water dripping into subterranean cavities in a fractured porous medium in order to improve estimates of dripping rates onto waste disposal canisters placed in caverns. These results suggest that conventional approaches and concepts, such as Darcy flow through a constant fracture aperture, may not be valid for characterizing unsaturated fractured rocks.Several laboratory-scale investigations were conducted using natural fracture cores and fracture replicas to investigate factors and processes affecting flow and transport under con- In 1996, three groups of 16 wells were drilled to install monitoring instrumentation, including vertical wells of series T (depths of 3-6.7 m), and inclined wells (270-45 ø from vertical) of series E and R (lengths of 18.3-22.9 m). In 1997, after interpretation of the results of the 1996 infiltration test, which showed a zone of preferential flow between wells R1 and R2, two additional inclined wells (R5 and R6) were drilled between existing R wells. Note that inclined boreholes were mostly drilled from the outside of the pond to reduce the disturbance to the pond surface and to intersect more vertical columnbounding fractures to better characterize the three-dimensional geometry of flow below the pond.The types of instrumentation used during the infiltration tests are summarized in Table 1. A more detailed description of the instrumentation is given by Faybishenko et al. [1998b]. All the instruments were installed in boreholes using the following method of borehole completion [Faybishenko et al., 1998d]. The instruments were attached to the outer faces of polyethylene packers mounted onto sections of 3.5-crn OD PVC manchette pipe, and the sections were glued together to form a continuous string of instruments placed at preselected depths. After the string with the probes was lowered into a borehole, the packers and the space between the packers were infilled with impermeable polyurethane resin, thus pressing the instruments against the borehole walls. This completion method ensured that no water flow occurred through the bore- small trench (15 cm high by 15 cm wide) was cut into the basalt at the location of the previous berm and a permanent concrete wall was constructed that practically eliminated water leakage beneath and through the berm. The experimental data and fitting curves are shown in Figure 6, and the fitting parameters for each test, which were determined using a lea...

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Abstract. The fracture distribution in basalt flows is a direct result of thermal processes. Thus basalt flows present a unique opportunity to characterize a nearly perfect deterministic system with its fundamental physical parameters. Fracture distribution data collected on cliff exposures of basalt flows near the Idaho National Engineering and Environmental Laboratory (INEEL) are combined with calculations of cooling rate and temperature distribution from a finite element model to construct a predictive methodology for fracture spacing. The methodology is based on an empirical power law relationship between inverse cooling rate and fracture spacing. The methodology may be applied to unexposed basalt flows of approximately elliptical cross section whose thickness and width are constrained only by geophysical or borehole data if sufficient fracture data on nearby exposed flows are available. The methodology aids waste remediation efforts at sites involving contaminant transport through fractured basalt, such as the INEEL and the Hanford site in Washington, as well as involving transport and fluid flow through volcanic or intrusive rocks where thermal processes are responsible for fracturing. IntroductionColumnar tensile fractures in basalt flows are driven by thermal elastic strain due to differential cooling of the basalt, as opposed to regional tectonic extension. Given the low matrix permeability of basalt, characterizing the spatial distribution of thermal contraction fractures is the key to understanding the overall permeability structure of multiple basalt flows. The goal of the geological portion of the integrated study, and the subject of this paper, is the development of a methodology to predict fracture distribution in basalt flows when only limited knowledge of the flow geometry is available. Obtaining fracture information on basalt flows not exposed at the surface is challenging, as geophysical methods have resolution falling below that required to image the fractures directly and core fracture measurements sample a very small volume. The We integrate field data on basalt flow geometry and fracture spacing with numerical constraints on the thermal history during fracturing. The result provides a methodology to estimate fracture spacing as a function of cooling rate, controlled by flow thickness and geometry. Although this work is part of a study for hazardous waste containment, aspects are directly applicable to water, oil, or gas reservoirs in thermally fractured volcanic or intrusive rocks.

A conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone was developed based on the results of lithological studies and a series of ponded infiltration tests conducted at the Box Canyon site near the Idaho National Engineering and Environmental Laboratory (INEEL) in Idaho. The infiltration tests included one two-week test in 1996, three two-day tests in 1997, and one four-day test in 1997. For the various tests, initial infiltration rates ranged from 4.1 cm/day to 17.7 cm/day and then decreased with time, presumably due to mechanical or microbiological clogging of fractures and vesicular basalt in the near-surface zone, as well as the effect of entrapped air. The subsurface moisture redistribution was monitored with tensiometers, neutron logging, time domain reflectrometry and ground penetrating radar. A conservative tracer, potassium bromide, was added to the pond water at a concentration of 3 gIL to monitor water flow with electrical resistivity probes and water sampling. Analysis of the data showed evidence of preferential flow rather than the propagation of a uniform wetting front. We propose a conceptual model describing the saturationdesaturation behavior of the basalt, in which rapid preferential flow through vertical column-bounding fractures occurs from the surface to the base of the basalt flow. After the rapid wetting of column-bounding fractures, a gradual wetting of other fractures and the basalt matrix occurs. Fractures that are saturated early in the tests may become desaturated thereafter, which we attribute to the redistribution of water between fractures and matrix. Lateral movement of water was also observed within a horizontal central fracture zone and rubble zone, which could have important implications for contaminant accumulation at contaminated sites.

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